1. Field of the Invention
The present invention relates to a valve device provided, for example, in an antiskid control system of a vehicle. More particularly, it relates to a valve device for opening and closing a passage connecting a master cylinder and a wheel cylinder.
2. Description of the Related Art
In a conventional antiskid control system, a construction in which a three-port-three-position valve is provided for increasing, holding, or decreasing a pressure in a wheel cylinder is known. Such a three-port-three-position valve has two valve elements for opening and closing two ports which communicate with the master cylinder and a reservoir for the brake oil. In a valve device described in Japanese Examined Utility Model Publication No. 58-17169, such a three-port-three-position valve has a construction in which a movable member supporting the two valve elements is moved according to an amount of electric current applied to a solenoid, a main spring being provided between the movable member and a casing, and a subspring being provided between one of the valve elements and the movable member. Thus, when an electric current is not applied to the solenoid, the movable member is at a first position in which one of the valve elements closes the corresponding port. Conversely, when a small electric current is applied to the solenoid, the movable member is at a second position against the main spring in which both valve elements close each corresponding port through the force of the subspring. Further, when a large electric current is applied to the solenoid, the movable member is at a third position against the main spring in which one of the above valve elements opens the corresponding port and the other valve element closes the corresponding port, by compressing the subspring.
As described above, in the conventional valve device, two kind of springs are needed, i.e., the main spring and the subspring, and the subspring must be provided between the two valve elements in the movable member. Therefore, the construction of the valve device is complicated. Further, in a construction wherein the two valve elements can take three positions, the spring constants of the main spring and the subspring, and the loads preset to the springs, must correspond precisely to the electromagnetic force of the solenoid.